1. Field of the Invention
The present invention relates to a fuel cell, and in particular, to a fuel cell having high CO resistance.
2. Description of the Related Art
In this age, the new technologies that have evolved, such as IT and biotechnology, have had a global impact. Even in such circumstances, however, the energy industry remains one of the largest basic industries. In recent years, as environmental awareness including prevention of global warming has grown, expectations regarding the introduction of a so-called new energy have increased. This new energy has advantages in terms of reduction in power transmission losses and security of power supply, in addition to environmental friendliness, given that the energy can be produced in dispersed sites close to the electrical power consumers. Furthermore, as a secondary effect, it is expected that new related industries will be created through the development of this new energy. Efforts to develop this new energy began in earnest, triggered by the oil crisis of approximately 30 years ago. At present, the following types of new energy are still at the development stage, but are moving toward practical use: reproducible energy produced by solar photovoltaic power generation or the like, recycled energy produced by waste power generation or the like, high efficiency energy produced by a fuel cell or the like, and energy in new fields, as typified by clean energy vehicles.
Among these examples, the energy produced by a fuel cell is one of the types receiving the most attention from industry. A fuel cell generates electricity and heat simultaneously through chemical reaction of oxygen in an atmosphere with hydrogen, produced through the reaction of water vapor with natural gas, methanol, or the like. A fuel cell produces only water as a by-product of power generation. In addition to this, high efficiency is obtained even in a low power output range, and the electrical power generation is not affected by weather, and therefore, is stable. In particular, the polymer electrolyte fuel cell has received significant attention as one of the next-generation standard power sources for applications such as use in vehicles, mobile use, and stationary use such as in housing. The following technologies have been developed for commercialization based on the polymer electrolyte fuel cell: a technology employing a small size catalyst of nanometer order, in order to improve power generation performance (see Published Japanese translation of PCT international application No. 2005-515063); and a technology in which gold nanoparticles are added to a catalyst in order to improve CO (carbon monoxide) resistance (see Koji Matsuoka, Kohei Miyazaki, Yasutoshi Iriyama, Takeshi Abe, and Zempachi Ogumi, “Methanol Oxidization Characteristics of Pt-Ru Catalyst Supported on Gold Ultra-fine Particles”, Proceedings of the 45th cell forum, the Committee of Battery Technology, the Electrochemical Society of Japan, (11/27 Heisei 16), pp. 620-621, and Kohei Miyazaki, Koji Matsuoka, Yasutoshi Iriyama, Takeshi Abe, and Zempachi Ogumi, “Electro-oxidation of Methanol on Gold Nanoparticles Supported on Pt/MoOx/C”, Journal of The Electrochemical Society, 152(9) A1870-A1873 (2005).
In the case where hydrogen is produced through the reaction of natural gas or methanol with water vapor as mentioned above, ideally, 80% of hydrogen (H2) and 20% of carbon dioxide (CO2) are supplied to a fuel cell through the reaction represented by the chemical equations (1) and (2). However, since carbon monoxide (CO) generated during the processes represented by the chemical equations (1) and (2) cannot be fully eliminated, CO in an amount of several ppm to several tens of ppm enters the anode of the fuel cell.CH4+H2O→3H2+CO  (1)CO+H2O→CO2+H2  (2)
Furthermore, in the case where an aqueous solution containing methanol (organic fuel) is supplied to a fuel cell, the reaction represented by the chemical equation (3) occurs on the anode side. However, CO which is not converted into carbon dioxide during the reaction process remains at the anode.CH3OH+H2O→6H++6e−+CO2  (3)
Hence, in a catalyst layer for the anode of a fuel cell which generates electrical power by means of organic fuel or reformed gas obtained through reforming reaction (transformation reaction), a catalyst such as platinum (Pt) which has a function of converting H2 to protons (H+) has usually been employed. In addition to platinum, a PtRu catalyst has also been employed to which ruthenium (Ru) is added in order to prevent the reduction of catalytic activity caused by CO poisoning of Pt. Materials such as ruthenium (Ru) have properties that promote the conversion of the CO that sticks to Pt to CO2. Furthermore, a technology has been reported which improves CO resistance by mixing a catalyst composed of Pt and Ru or Pt and molybdenum (Mo) with gold nanoparticles. However, this technology is still in the research stage. Although fuel cells appear on the verge of becoming genuinely widespread, it has been found that, at present, the CO resistance at the anode is not sufficient.